IMO 2006 Problem 3

Determine the least real number $M$ such that the inequality $$|ab(a^{2}-b^{2})+bc(b^{2}-c^{2})+ca(c^{2}-a^{2})| \leq M(a^{2}+b^{2}+c^{2})^{2}$$ holds for all real numbers $a$ , $b$ and $c$ .